1. Field of the Invention
The present invention relates to filter circuits and is more particularly concerned with an integratable filter circuit which has a bandpass filter characteristic.
2. Description of the Prior Art
Integrated circuits for analog signal processing frequently contain filter circuits in order to suppress undesired disturbances. Therefore, bandpass filters are very frequently employed, for example for filtering out narrow band useful signals from broadband noise signals. The filter circuits are thereby preferably designed such that an optimally low, external wiring expense is needed for setting the center frequency. Since suitable capacitances and inductances were heretofore very difficult to realize in integrated technology or, respectively, could not be realized at all in integrated technology, external elements, particularly capacitors, are usually utilized as elements for setting the center frequency.
For example, bandpass filters having a network composed of two resistors and two capacitors in the feedback loop of an operational amplifier are known from U. Tietze and Ch. Schenk, Halbleiterschaltungstechnik, 5th Edition 1980, pp. 300 et sec, whereby a maximum of one of the two capacitors is connected to the reference potential. Since the operational amplifier as well as the two resistors can be realized in integrated technology, only the two capacitors have to be externally connected thereto. To that end, however, at least three further terminals must be provided for the circuit under consideration.